A conjugated diolefin rubber exhibiting excellent abrasion resistance, rupture characteristics, and grip characteristics as well as superior operational stability (e.g., wet skid resistance) has been desired as a tire material in order to deal with a recent demand for safety and operational stability of vehicles.
As a rubber material with a low hysteresis loss, natural rubber, polyisoprene rubber, polybutadiene rubber, and the like have been known. However, these rubber materials exhibit poor wet skid resistance.
As a method for decreasing hysteresis loss without impairing wet skid resistance, a method of introducing functional groups into the terminals of styrene-butadiene copolymers having various structures and polymerized in a hydrocarbon solvent using an organolithium initiator has been proposed. A styrene-butadiene copolymer obtained by modifying or coupling the polymer terminal with a tin compound (see Patent Document 1) and a styrene-butadiene copolymer obtained by modifying the polymer terminal with an isocyanate compound (see Patent Document 2) have been known. These modified polymers exhibit a reduced hysteresis loss and excellent abrasion resistance and rupture characteristics without showing reduced wet skid resistance particularly when using a composition containing carbon black as a reinforcing agent.
On the other hand, a method of using a rubber composition containing silica or a mixture of silica and carbon black as a reinforcing agent as a rubber material for tires has been proposed recently. A tire tread formed using silica or a mixture of silica and carbon black exhibits low rolling resistance and provides excellent operational stability represented by wet skid resistance. However, a vulcanizate thereof exhibits a low tensile strength and abrasion resistance. The above modified styrene-butadiene copolymer can produce an excellent rubber material for tires exhibiting superior abrasion resistance and rupture characteristics if carbon black is used as a reinforcing agent, but the effect is small when silica is used as a reinforcing agent.
In order to improve the tensile strength and the abrasion resistance of a vulcanizate containing silica or a mixture of silica and carbon black, a rubber composition containing a polymer into which a functional group having affinity with silica is introduced has been proposed. A method for producing a polymer by reacting silicon tetrahalide, trihalosilane, or the like has been proposed in Patent Document 3. Patent Document 4 discloses a method of producing a polymer modified by a halogenated silane compound. Patent Document 5 discloses a diene rubber with an alkyl silyl group introduced therein and Patent Document 6 discloses a diene rubber with a halogenated silyl group introduced therein. Patent Document 7 discloses a diene rubber into which a tertiary amino group and an alkoxysilyl group are introduced.
Although some improvement in properties can be seen by using these modified polymers in the composition comprising silica or a mixture of silica and carbon black, improvement in tensile strength and abrasion resistance of the vulcanizate is not sufficient. In addition, the effect of lowering the hysteresis loss is not necessarily satisfactory in the composition using a mixture of silica and carbon black, particularly when the proportion of carbon black is increased. Moreover, a silica-containing composition exhibits inferior processability as compared with a carbon black-containing composition, resulting in an increased processing cost. Processability tends to become worse when a polymer in which the above-described functional groups having affinity with silica are introduced.
Generally known modified polymers are classified into those suitable for use mainly with carbon black and those suitable for use with silica. When altering the type of a reinforcing agent used in manufacturing tires, rubbers to be used must be reselected. In addition, when a mixture of silica and carbon black is added, the effect obtained has a correlation with the ratio of silica and carbon black irrespective of the type of the modified polymer used.
Polymers modified by introducing an amino group are thought to be effective in either a composition to which carbon black is added or a composition to which silica is added. Examples of such polymers for use with carbon black that have been proposed include (1) polymers obtained by introducing an amino group into the polymer terminal using a lithium amide initiator (see Patent Documents 8 to 12) and (2) styrene-butadiene copolymers having different structures polymerized using an organolithium initiator, of which the terminals are modified with a nitrogen-containing compound such as a urea compound (see Patent Document 13), a dialkylamino benzophenone compound (see Patent Documents 14 and 15), or a lactam compound (see Patent Document 16). Diene rubbers with an amino group introduced therein have been disclosed as polymers for use with silica (see Patent Documents 17 to 19).
A certain degree of improvement in the properties has been achieved by the modified polymers obtained using these methods in both the compositions containing silica and the compositions containing carbon black. Although the above prior art contain detailed descriptions of the method of introducing an amino group, the prior art only disclose general information concerning the relationship between the structure and performance of the polymers.
As a novel conjugated diolefin copolymer rubber having good processability and well-balanced low hysteresis loss, abrasion resistance, and rupture characteristics without impairing wet skid properties which is used together with either carbon black or silica, the inventors of the present invention have previously proposed a copolymer rubber of a conjugated diolefin and an aromatic vinyl compound having a primary amino group and an alkoxysilyl group which are bonded to the copolymer chain (see Patent Documents 20 and 21). Rubber compositions having various improved rubber characteristics mentioned above can be obtained by using the proposed technology. However, in order to further improve operational stability and safety of vehicles, a copolymer rubber having excellent wet skid properties, abrasion resistance, a low rebounding coefficient, and road surface gripping characteristics in both dry and rainy conditions (dry grip and wet grip) has been desired.    Patent Document 1: JP-A-57-55912    Patent Document 2: JP-A-61-141741    Patent Document 3: JP-B-49-36957    Patent Document 4: JP-B-52-5071    Patent Document 5: JP-A-1-188501    Patent Document 6: JP-A-5-230286    Patent Document 7: JP-A-7-233217    Patent Document 8: JP-A-59-38209    Patent Document 9: JP-B-5-1298    Patent Document 10: JP-A-6-279515    Patent Document 11: JP-A-6-199923    Patent Document 12: JP-A-7-53616    Patent Document 13: JP-A-61-27338    Patent Document 14: JP-A-58-162604    Patent Document 15: JP-A-58-189203    Patent Document 16: JP-A-61-43402    Patent Document 17: JP-A-1-101344    Patent Document 18: JP-A-64-22940    Patent Document 19: JP-A-9-71687    Patent Document 20: JP-A-2003-171418    Patent Document 21: JP-A-2004-67987